For as long as I can remember, I’ve been attracted to the History of Radio Electronics – the companies who built radio products and the people who designed them. Yet, going steps deeper, my curiosity involves the components that make an equipment’s various electronic circuits work and the people behind them.
Perhaps you have the same curiosity? Bet so.
If you are a Ham Radio operator and got your first license in the 1960s, then you remember the intriguing advertisements in QST, CQ, and Popular Electronics magazines. For me, I was drawn to the majestic HF transmitters made by the Technical Materiel Corporation (TMC). Who while operating their 50 watt Novice Class transmitter of the ‘peanut whistle’ variety didn’t dream of the ionospheric excitement one of those TMC 40-Kilowatt rigs could create? Others who were involved in the United States Navy or Air Force no doubt grew an affinity to the high quality rigs developed by the Collins Radio Company of Cedar Rapids, Iowa.
In time, it happens: one of these treasured rigs finds a home in the workshop and the desire to restore it to its former operational glory takes root.
Okay….where do ya start? How do you make these sets look so good and work so brilliantly? Those are the two questions I get asked endlessly. There’s no short answer, but we’ll start with the basics here and grow the discussion over a series in this Blog space.
First off…DO NOT power your as-discovered treasure up! It likely has been decommissioned and unpowered for many years. Its circuitry, particularly the DC rectifier stage, likely includes electrolytic filter capacitors. These gems utilize a chemical process to develop a thin insulating barrier or oxide within the capacitor’s “plates”. Check the Internet and you’ll find many explanations on how electrolytic capacitors are made to function. In short (no pun intended) a DC bias voltage is impressed across the capacitor and the formation of its insulating oxide begins.
Yet, if no DC voltage is present and over long periods, the oxide layer dissipates/breaks down, and this is where the problem starts. If one was to suddenly apply voltage to a capacitor in such as state, it will likely overheat, rupture (with a very loud explosion) and -- in the process -- damage the rectifiers (tubes or semiconductor diodes) or, far worse, damage the power transformer. Specialized, custom-made power transformer do not grow on trees and are expensive to replace...assuming they can be. Take it easy.
In this series, we’ll get into the process of safely powering up long dormant electronic items so stay tuned. But before diving in blind, you must gain proficiency on general workshop basics and the manner in which your treasure is supposed to operate. When I owned a radio service company, the most frustrating part of the job was getting our technicians to take that important step back and to study the equipment’s service manual/schematic diagrams. Unless one knows how the device should function, you can’t possibly know if it is truly repaired and 100% functional. Guessing only wastes time, wastes money -- which typically results in frustration and a still-dead radio.
If you’re still reading this, I’m assuming you have some fundamental knowledge of electronics. If not, an excellent book (albeit older) is Electronic Communications by Robert Shrader. Other important books on radio fundamentals that should find a place in your personal library include Electronic and Radio Engineering by Frederick Terman; RCA Radiotron Designer’s Handbook - 4th Edition; Radio Handbook by Bill Orr; and Radio Amateur’s Handbook by the American Radio Relay League. Of course, you should have a ready set of electron tube handbooks such as those published years ago by General Electric, Sylvania, and the Radio Corporation of America (RCA). These each provide valuable information on tube operational parameters that will become important resources during your project’s parts evaluation, replacement, and troubleshooting phases. Reprints are readily available so don’t fret on how to find these oldies.
In my work restoring vintage radio/electronic equipment, I’ve found the following to be 100% essential:
· An Analog Oscilloscope. 100MHz bandwidth is fine, but 300MHz is best. I use older Tektronix 465/467 as well as 7000-series mainframe scopes. These were designed in the era of hybrid electronics where both high voltages for tubes and low voltages for solid state devices were common. Newer digital ‘scopes are okay, but I’m an analog purist.
· An Analog High-Impedance Voltmeter. In older days these were termed vacuum tube voltmeters. As solid state electronics gradually pushed aside tubes, this meter got a new active element: the Field Effect Transistor and so these became also known as FET Voltmeters. Obviously, they each measure voltage amplitudes, both AC and DC, but also provide a means for measuring resistance (Ohms).
· A Digital Voltmeter. Important in measuring absolute values of voltage, resistance, etc. yet cannot replace an analog meter for ease of determining resonance for tuned circuits. Digital meters usually have a far lower input impedance than the two cited above. Consequently, they should not be used on RF tuned circuits and their low input impedance will load down a tuned circuit, thereby degrading selectivity (commonly referred to as a Q-Killer). Don’t know what Q is? Time to hit the books as Q or Quality Factor is important.
· RF Signal Generator. This should cover the range of 100KHz to at least 30MHz if your interest is in high-frequency radio. Higher, of course, if the project so dictates. Your RF signal generator should have a precision output level attenuator, metered level indicators, internal AM modulation capability, and an accurate frequency readout. An inexpensive but highly rugged instrument is the HP-8740 series or even the older HP606 series unit.
· AF Signal Generator. Some work will likely involve the application of audio levels to test modulation circuits or internal audio detector/amplifier circuitry. Simple units here will suffice but should be tunable from as low as 30Hz to as high as 100KHz. Metering is not necessary but is desirable. Yet, if no internal metering is available, we can turn to that trusty oscilloscope or those external voltmeters, as described earlier.
· Digital Capacitor/Inductor Meter. These are inexpensive but useful devices to characterize physical components. Capacitors, for example, use many different means to indicate value. Armed with one of these measuring gems, one can quickly determine a part’s value and perhaps prevent the inadvertent installation of a wrong-value part. One can’t imagine how much time is often wasted fishing out one of those mistakes! The only good that comes out of a wrong-value problem is one’s heightened knowledge of electronic theory – gained while valiantly trying to figure out WHY things aren’t working out as planned.
· Transconductance Tube Tester. Beware there are two types of tube testers out there, folks. One type merely measures cathode emission whereas the other actually tests tubes for amplification (transconductance) properties. Or as one old timer once coldly declared to my formerly young self, ‘Just because it glows don’t mean a damned thing.’ So true.
· Isolation transformer. This is essential for any radio/electronic device that lacks an internal power transformer. Some low-cost devices were powered directly from the AC line/socket. These are potential shock hazards which can be mitigated via an isolation transformer.
· Metered Variac/Autotransformer. This device allows one to gradually apply power to a newly restored electronic item. The metering function, which should measure both applied voltage and load current, serves as an immediate notice of impending doom. If, while slowly applying line voltage to a device, the load current shoots up like a rocket that indicates: a wiring error, internal short circuit, bad filter capacitor, shorted filter choke, etc. We’ll discuss later a ‘divide and conquer’ approach to isolating these sorts of defects.
· Tools/Small Parts. An assortment of common hand tools, metal working files, and various type of electronic hardware (i.e., nuts, bolts, grommets, wire lugs, and washers).
Of course, as you progress in the hobby of equipment restoration, other more specialized pieces of test gear and tools will gradually inhabit your workshop. Now, please don’t laugh, but two of my most important restoration tools have absolutely nothing to do with the operation of functionality of electronics. They have to do with aesthetics.
Now, think about it. Who would want to claim a nasty, rusty, dirt-caked, paint-faded hulk as a prized possession? Or let it occupy a valued spot in one’s home or radio shack??
Nobody is who.
A restored radio or electronic device must look great as well as operate as designed. So, a restorer should work to gain confidence and practical experience in cabinet refurbishment, front-panel lettering, and….the worst…painting! So, what are those two important tools I can't live without??
Here ya go: an air compressor and bead blasting cabinet!
See you next time as we dig deeper.